Fix grammar in Text Splitters docs (#4373)

# Fix grammar in Text Splitters docs

Just a small fix of grammar in the documentation:

"That means there two different axes" -> "That means there are two
different axes"

docs/modules/indexes/text_splitters.rst

@@ -15,7 +15,7 @@ At a high level, text splitters work as following:
 2. Start combining these small chunks into a larger chunk until you reach a certain size (as measured by some function).
 3. Once you reach that size, make that chunk its own piece of text and then start creating a new chunk of text with some overlap (to keep context between chunks).
 
-That means there two different axes along which you can customize your text splitter:
+That means there are two different axes along which you can customize your text splitter:
 
 1. How the text is split
 2. How the chunk size is measured