Add quotes to the command for the INPUT_FILTER doxygen setting.

This allows older versions of Doxygen (tested with 1.8.14) to apply
the filter correctly.

doc/CodeDocumentation.conf.in

@@ -1131,7 +1131,7 @@ IMAGE_PATH             =
 # characters, include newline. If present, Doxygen-style LaTeX commands
 # such as \f$, \f[, etc., are left unchanged.
 
-INPUT_FILTER           = perl -0777 -pe 's/\$\$((?:.|\n)+?)\$\$/\\f[\1\\f]/g; s/(?<!\\f)\$((?:.|\n)+?)\$/\\f\$\1\\f\$/g'
+INPUT_FILTER           = "perl -0777 -pe 's/\$\$((?:.|\n)+?)\$\$/\\f[\1\\f]/g; s/(?<!\\f)\$((?:.|\n)+?)\$/\\f\$\1\\f\$/g'"
 
 # The FILTER_PATTERNS tag can be used to specify filters on a per file pattern
 # basis. Doxygen will compare the file name with each pattern and apply the

fem/hybridization.hpp

@@ -19,7 +19,8 @@
 namespace mfem
 {
 
-/** Auxiliary class Hybridization, used to implement BilinearForm hybridization.
+/** @brief Auxiliary class Hybridization, used to implement BilinearForm
+    hybridization.
 
     Hybridization can be viewed as a technique for solving linear systems
     obtained through finite element assembly. The assembled matrix $ A $ can