Fix minor typo in usage string of grade cmd

Author: madhavarshney

bots/py/commands/grade.py

@@ -12,7 +12,7 @@
 async def grade(ctx, *, msg: str = " "):
 
     prof_and_class = msg.split(" for ")
-    usage_str = f'**Usage:** \n`1. {ctx.prefix}grade professor for subject [course]"`\n`2. {ctx.prefix}grade professor for everything`'
+    usage_str = f'**Usage:** \n`1. {ctx.prefix}grade professor for subject [course]`\n`2. {ctx.prefix}grade professor for everything`'
 
     if len(prof_and_class) != 2:
         await ctx.channel.send(usage_str)