The 'bindu' or 'anuswaar' (the dot on top of the ल in लंबे) has a pronunciation that depends on the letter that that comes after it. It is typically pronounced as one of the five nasal consonants- ङ्, ञ्, ण्, न् or म्. For instance, it is म् in लंबे, न् in हिंदी and so on. Consequently, it is correct to write लंबे as लम्बे or हिंदी as हिन्दी. Most modern style guides prefer the form with the dot (especially for the letters ङ् and ञ् which are extremely rare in Hindi outside of such anuswaar pronunciation) but you can also use the alternative.