grammar fix

Doc/lib/librunpy.tex

@@ -10,7 +10,7 @@
 \versionadded{2.5}
 
 The \module{runpy} module is used to locate and run Python modules
-without importing them first. It's main use is to implement the
+without importing them first. Its main use is to implement the
 \programopt{-m} command line switch that allows scripts to be located
 using the Python module namespace rather than the filesystem.