Fix doc formatting

Author: js-krinay

pgb/add.py

@@ -1,6 +1,6 @@
 def add(a: int, b: int) -> int:
     """
-    Adds two number
+    Adds two number.
 
     Args:
         a (int): First argument

pgb/mul.py

@@ -1,6 +1,6 @@
 def mul(a: int, b: int) -> int:
     """
-    Multiplies two number
+    Multiplies two number.
 
     Args:
         a (int): First argument

pgb/pow.py

@@ -1,6 +1,6 @@
 def pow(a: int, b: int) -> int:
     """
-    Power of two number
+    Power of two number.
 
     Args:
         a (int): First argument

pgb/sub.py

@@ -1,6 +1,6 @@
 def sub(a: int, b: int) -> int:
     """
-    Subtracts two number
+    Subtracts two number.
 
     Args:
         a (int): First argument