Update base/doc/usrguide.tex

Co-authored-by: Yukai Chou <[email protected]>

base/doc/usrguide.tex

@@ -1271,7 +1271,7 @@ \section{Support for problem solving}
 will (hopefully) include the descriptions, dates and version numbers
 for package and class files.
 
-Sometimes, if may be that a local edit has been made to a package or
+Sometimes, it may be that a local edit has been made to a package or
 class file (or rather a copy of such a file). To allow these cases to
 be identified, \cs{listfiles} takes an optional argument which allows
 adjustment of the information printed using a key--value approach