Use correct method insert_text_after.

Afaik the `insert_after` does not work, when i changed my code to use `insert_text_after` 
it worked. So changed the documentation to match that.

Author: nathanvda

README.md

@@ -68,7 +68,7 @@ require 'docx'
 doc = Docx::Document.open('example.docx')
 
 # Insert a single line of text after one of our bookmarks
-doc.bookmarks['example_bookmark'].insert_after("Hello world.")
+doc.bookmarks['example_bookmark'].insert_text_after("Hello world.")
 
 # Insert multiple lines of text at our bookmark
 doc.bookmarks['example_bookmark_2'].insert_multiple_lines_after(['Hello', 'World', 'foo'])
@@ -104,4 +104,4 @@ p_child = p_element.at_xpath("//child::*") # selects first child
 * Default formatting of inserted elements to inherited values
 * Implement formattable elements.
 * Implement styles.
-* Easier multi-line text insertion at a single bookmark (inserting paragraph nodes after the one containing the bookmark)
+* Easier multi-line text insertion at a single bookmark (inserting paragraph nodes after the one containing the bookmark)