docs(cogs): add necessary ctx parameter to math command examples (#491)

Signed-off-by: bmzr <[email protected]>

Author: bmzr

docs/popular-topics/cogs.mdx

@@ -44,12 +44,12 @@ class Greetings(commands.Cog): # create a class for our cog that inherits from c
     )
 
     @math.command()
-    async def add(self, a: int, b: int):
+    async def add(self, ctx, a: int, b: int):
         c = a + b
         await ctx.respond(f"{a} + {b} is {c}.")
 
     @advanced_math.command()
-    async def midpoint(self, x1: float, y1: float, x2: float, y2: float):
+    async def midpoint(self, ctx, x1: float, y1: float, x2: float, y2: float):
         mid_x = (x1 + x2)/2
         mid_y = (y1 + y2)/2
         await ctx.respond(f"The midpoint between those coordinates is ({mid_x}, {mid_y}).")