Fix typo in doc (#6313)

Author: CzBiX

doc/PROGMEM.rst

@@ -116,7 +116,7 @@ the ``F()`` macro and possibly the FPSTR() macro. These are defined in `WString.
     #define FPSTR(pstr_pointer) (reinterpret_cast<const __FlashStringHelper *>(pstr_pointer))
     #define F(string_literal) (FPSTR(PSTR(string_literal)))
 
-So ``FSPTR()`` takes a PROGMEM pointer to a string and casts it to this
+So ``FPSTR()`` takes a PROGMEM pointer to a string and casts it to this
 ``__FlashStringHelper`` class. Thus if you have defined a string as
 above ``xyz`` you can use ``FPSTR()`` to convert it to
 ``__FlashStringHelper`` for passing into functions that take it.