Update sheets.py

Added a missing .lower() to the line

cheat/sheets.py

@@ -84,7 +84,7 @@ def search(term):
     for cheatsheet in sorted(get().items()):
         match = ''
         for line in open(cheatsheet[1]):
-            if lowered_term in line:
+            if lowered_term in line.lower():
                 match += '  ' + line
 
         if match != '':