Add double backquotes for rst code samples (#1298)

Appends to (#1246)

Co-authored-by: kolibril13 <[email protected]>

Author: RickyC0626

manim/mobject/value_tracker.py

@@ -66,12 +66,12 @@ def increment_value(self, d_value):
         self.set_value(self.get_value() + d_value)
 
     def __iadd__(self, d_value):
-        """adds `+=` syntax to increment the value of the ValueTracker"""
+        """adds ``+=`` syntax to increment the value of the ValueTracker"""
         self.increment_value(d_value)
         return self
 
     def __isub__(self, d_value):
-        """adds `-=` syntax to decrement the value of the ValueTracker"""
+        """adds ``-=`` syntax to decrement the value of the ValueTracker"""
         self.increment_value(-d_value)
         return self