doc: fix maint1.tex LaTeX errors so it compiles (docs 28/28)

maint1.tex failed to build -- it was the one document that never
regenerated -- because of three source-level LaTeX errors, each masked by
the one before it:

  * `gdb program $pid`: a bare `$` opens math mode (meant literally), which
    was never closed, so `\endi` hit "Missing $ inserted" / Emergency stop.
    Escaped to `\$pid`.
  * `{\itshape process_ID}` (x2): a bare `_` is a math-mode subscript in
    text.  Escaped to `process\_ID`.
  * a tabbing block (`\ii ps -C wish` before `\ii gdb ...`, and the
    `gdb wish / run / set argc / set argv / source` block) was missing its
    `\\` row separators, so multiple `\ii` (= \>\>\>) accumulated in one row
    and overran the 4 tab stops -> "Undefined tab position".  Added the
    missing `\\`.

maint1 now compiles cleanly (two-pass, 0 errors) and `make all` in
doc/latexfiles produces 28/28 PostScript files instead of 27/28.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

doc/latexfiles/maint1.tex

@@ -601,7 +601,7 @@ magic under a debugger is the following:
 
 \starti
   \ii set pid=`ps --no-headers -o pid= -C wish` \\
-  \ii gdb program $pid
+  \ii gdb program \$pid
 \endi
 
 which is a simple invocation for Linux systems.  If it is not
@@ -610,11 +610,11 @@ process ID can be obtained visually from the {\bf ps} command
 and given to {\bf gdb} as the process ID to attach to:
 
 \starti
-  \ii ps -C wish
+  \ii ps -C wish \\
-  \ii gdb program {\itshape process_ID}
+  \ii gdb program {\itshape process\_ID}
 \endi
 
-wher the first command returns {\itshape process_ID}, the  process ID of
+wher the first command returns {\itshape process\_ID}, the  process ID of
 the {\bfseries wish} process.  If the
 version of {\bfseries ps} on your system does not support the {\bfseries
 -C} option, there are other options that will report the {\bfseries wish}
@@ -629,10 +629,10 @@ procedure to attach to the process prior to initialization is the
 following:
 
 \starti
-  \ii gdb wish
+  \ii gdb wish \\
-  \ii run
+  \ii run \\
-  \ii set argc 3
+  \ii set argc 3 \\
-  \ii set argv \{-w -d OGL\}
+  \ii set argv \{-w -d OGL\} \\
   \ii source /usr/local/lib/magic/tcl/magic.tcl
 \endi