Fix typo in primitive_docs.rs

Author: programmerjake

library/core/src/primitive_docs.rs

@@ -1251,7 +1251,7 @@ mod prim_f16 {}
 ///   - **Unchanged NaN propagation**: The quiet bit and payload are copied from any input operand
 ///     that is a NaN. If the inputs and outputs do not have the same size (i.e., for `as` casts), the
 ///     same rules as for "quieting NaN propagation" apply, with one caveat: if the output is smaller
-///     than the input, droppig the low-order bits may result in a payload of 0; a payload of 0 is not
+///     than the input, dropping the low-order bits may result in a payload of 0; a payload of 0 is not
 ///     possible with a signaling NaN (the all-0 significand encodes an infinity) so unchanged NaN
 ///     propagation cannot occur with some inputs.
 ///   - **Target-specific NaN**: The quiet bit is set and the payload is picked from a target-specific