It's an inverted caret.
The caret was originally and continues to be used in handwritten form as a proofreading mark to indicate where a punctuation mark, word, or phrase should be inserted into a document. The term comes from the Latin caret, "it lacks", from carēre, "to lack; to be separated from; to be free from". The caret symbol is written below the line of text for a line-level punctuation mark, such as a comma, or above the line as an inverted caret (cf. U+02C7 ˇ CARON) for a higher character, such as an apostrophe; the material to be inserted may be placed inside the caret, in the margin, or above the line.
- Wikipedia
If you're using a word processor, it might be easier to insert a comment with the correction, rather than a traditional proofreader mark.