doc/user: fix fragile LaTeX in speckley.tex (Debian #1122465)

Modern/strict TeX Live (as on Debian forky/sid) rejects the old LaTeX syntax in
the speckley weak-form equation, which contributed to the FTBFS in Debian bug
#1122465 (doc PDFs not produced -> dh_install missing files). Two visual no-ops:
- move the \displaystyle{...} closing brace so both integrals are inside it
  (the fix suggested by Santiago Vila on the bug), and
- remove a blank line inside the math `array` (a paragraph break in math mode,
  which stricter TeX Live errors on; TeX Live 2023 tolerated it).

user.tex builds a clean 150-page PDF on TeX Live 2023 with this change; a sweep
of doc/user/*.tex found no other blank-lines-in-math or $$ display math.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: Lutz Gross

doc/user/speckley.tex

@@ -38,8 +38,7 @@ \section{Formulation}
 \begin{array}{cl} &
 \displaystyle{
 \int_{\Omega}
-D \cdot vu \; d\Omega } + \int_{\Gamma} d \cdot vu \; d{\Gamma}
-
+D \cdot vu \; d\Omega + \int_{\Gamma} d \cdot vu \; d{\Gamma} }
 = \displaystyle{\int_{\Omega}  X_{j} \cdot v_{,j}+ Y \cdot v \; d\Omega }
 + \displaystyle{\int_{\Gamma} y \cdot v \; d{\Gamma}}
 \end{array}