chore: fix grammar

Author: crisbeto

src/material/datepicker/date-range-input-parts.ts

@@ -189,7 +189,7 @@ const _MatDateRangeInputBase:
   ]
 })
 export class MatStartDate<D> extends _MatDateRangeInputBase<D> implements CanUpdateErrorState {
-  /** Validator that checks whether the start date isn't after the end date. */
+  /** Validator that checks that the start date isn't after the end date. */
   private _startValidator: ValidatorFn = (control: AbstractControl): ValidationErrors | null => {
     const start = this._getValidDateOrNull(this._dateAdapter.deserialize(control.value));
     const end = this._model ? this._model.selection.end : null;
@@ -252,7 +252,7 @@ export class MatStartDate<D> extends _MatDateRangeInputBase<D> implements CanUpd
   ]
 })
 export class MatEndDate<D> extends _MatDateRangeInputBase<D> implements CanUpdateErrorState {
-  /** Validator that checks whether the end date isn't before the start date. */
+  /** Validator that checks that the end date isn't before the start date. */
   private _endValidator: ValidatorFn = (control: AbstractControl): ValidationErrors | null => {
     const end = this._getValidDateOrNull(this._dateAdapter.deserialize(control.value));
     const start = this._model ? this._model.selection.start : null;