Misc-typos (#741)

Found in downstream CMake repo via `codespell -q 3`
diff --git a/doc/doxyfile.in b/doc/doxyfile.in
index 7bbe9bc..baf04c1 100644
--- a/doc/doxyfile.in
+++ b/doc/doxyfile.in
@@ -1072,7 +1072,7 @@
 # defined cascading style sheet that is included after the standard style sheets
 # created by doxygen. Using this option one can overrule certain style aspects.
 # This is preferred over using HTML_STYLESHEET since it does not replace the
-# standard style sheet and is therefor more robust against future updates.
+# standard style sheet and is therefore more robust against future updates.
 # Doxygen will copy the style sheet file to the output directory. For an example
 # see the documentation.
 # This tag requires that the tag GENERATE_HTML is set to YES.