little fix in docstrin (create_heat_exchanger)

[jkisse]

closes #277

[codecov]

Codecov Report

Merging #279 (cbc9630) into develop (17aad63) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #279   +/-   ##
========================================
  Coverage    90.67%   90.67%           
========================================
  Files           74       74           
  Lines         3784     3784           
========================================
  Hits          3431     3431           
  Misses         353      353           

Impacted Files	Coverage Δ
pandapipes/create.py	99.02% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 17aad63...cbc9630. Read the comment docs.