Fix typo in LLVM Kaleidoscope tutorial (Chapter 2) (llvm#169319)

This patch fixes a minor typo in the **Kaleidoscope tutorial (Chapter
2)**.

The sentence:
“checks to see if **if** is too low”
has been corrected to:
“checks to see if **it** is too low”.

This is a documentation-only change and does not affect any semantic
behavior or code generation.

Thank you for maintaining the tutorial, and please let me know if any
further adjustments are needed.

Author: baramatsubonzo

llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl02.rst

@@ -453,7 +453,7 @@ starts with:
           return LHS;
 
 This code gets the precedence of the current token and checks to see if
-if is too low. Because we defined invalid tokens to have a precedence of
+it is too low. Because we defined invalid tokens to have a precedence of
 -1, this check implicitly knows that the pair-stream ends when the token
 stream runs out of binary operators. If this check succeeds, we know
 that the token is a binary operator and that it will be included in this