Fixed M48 mismatch between n and P parameter on documentation

Marlin/Marlin_main.cpp

  *        Call gcode file : "M32 P !filename#" and return to caller file after finishing (similar to #include).
  *        The '#' is necessary when calling from within sd files, as it stops buffer prereading
  * M42  - Change pin status via gcode Use M42 Px Sy to set pin x to value y, when omitting Px the onboard led will be used.
- * M48  - Measure Z_Probe repeatability. M48 [n # of points] [X position] [Y position] [V_erboseness #] [E_ngage Probe] [L # of legs of travel]
+ * M48  - Measure Z_Probe repeatability. M48 [P # of points] [X position] [Y position] [V_erboseness #] [E_ngage Probe] [L # of legs of travel]
  * M80  - Turn on Power Supply
  * M81  - Turn off Power Supply
  * M82  - Set E codes absolute (default)
    * M48: Z-Probe repeatability measurement function.
    *
    * Usage:
-   *   M48 <n#> <X#> <Y#> <V#> <E> <L#>
+   *   M48 <P#> <X#> <Y#> <V#> <E> <L#>
    *     P = Number of sampled points (4-50, default 10)
    *     X = Sample X position
    *     Y = Sample Y position
    * as been issued prior to invoking the M48 Z-Probe repeatability measurement function.
    * Any information generated by a prior G29 Bed leveling command will be lost and need to be
    * regenerated.
-   *
-   * The number of samples will default to 10 if not specified.  You can use upper or lower case
-   * letters for any of the options EXCEPT n.  n must be in lower case because Marlin uses a capital
-   * N for its communication protocol and will get horribly confused if you send it a capital N.
    */
   inline void gcode_M48() {
 
     if (verbose_level > 0)
       SERIAL_PROTOCOLPGM("M48 Z-Probe Repeatability test\n");
 
-    if (code_seen('P') || code_seen('p') || code_seen('n')) { // `n` for legacy support only - please use `P`!
+    if (code_seen('P') || code_seen('p')) { // `n` for legacy support only - please use `P`!
       n_samples = code_value_short();
       if (n_samples < 4 || n_samples > 50) {
         SERIAL_PROTOCOLPGM("?Sample size not plausible (4-50).\n");