sentence-end-double-space

sentence-end-double-space is a variable defined in `paragraphs.el'.
Its value is

t

• This variable is safe as a file local variable if its value
  satisfies the predicate `booleanp'.

Documentation:
Non-nil means a single space does not end a sentence.
This is relevant for filling. See also `sentence-end-without-period'
and `colon-double-space'.

This value is used by the function `sentence-end' to construct the
regexp describing the end of a sentence, when the value of the variable
`sentence-end' is nil. See Info node `(elisp)Standard Regexps'.

You can customize this variable.