fix: harfrq command (#3729)

* fix: harfrq command

* chore: adding changelog file 3729.fixed.md [dependabot-skip]

---------

Co-authored-by: pyansys-ci-bot <[email protected]>

Author: germa89

doc/changelog.d/3729.fixed.md

@@ -0,0 +1 @@
+fix: harfrq command

src/ansys/mapdl/core/_commands/solution/dynamic_options.py

@@ -261,7 +261,7 @@ def harfrq(self, freqb="", freqe="", logopt="", freqarr="", toler="", **kwargs):
 
         This command is also valid in PREP7.
         """
-        command = f"HARFRQ,{freqb},{freqe},{logopt},{freqarr},{toler}"
+        command = f"HARFRQ,{freqb},{freqe},,{logopt},{freqarr},{toler}"
         return self.run(command, **kwargs)
 
     def hrexp(self, angle="", **kwargs):