Typo fix for decimal docs

Doc/lib/libdecimal.tex

@@ -453,7 +453,7 @@ active context.
   no context is specified, a copy of the current context is used.
   \versionadded{2.5}
 
-  For example, the following code set the current decimal precision
+  For example, the following code sets the current decimal precision
   to 42 places, performs a calculation, and then automatically restores
   the previous context:
 \begin{verbatim}